1) Field of the Invention
The present invention relates to an optical transmission system. In particular, the present invention relates to an optical transmission system which performs WDM (Wavelength Division Multiplex) transmission of optical signals.
2) Description of the Related Art
The WDM technology is a widely used, core technology for optical transmission systems. According to WDM, signals in a plurality of channels are concurrently transmitted through a single optical fiber by multiplexing light having different wavelengths. In WDM systems, optical supervisory signals having a bandwidth of about 1.5 to 150 Mb/s and being called OSC (Optical Supervisory Channel) signals are transmitted as well as main signals having a bandwidth of 2.4 to 40 Gb/s.
The OSC signals are optical signals used for detection of troubles in transmission lines as well as condition monitoring and setting for administration, e.g., condition monitoring and setting control of optical amplifiers. Therefore, normally, in the WDM systems, only the main signals are amplified by optical amplifiers (e.g., erbium-doped-fiber amplifiers, which are hereinafter referred to as EDFAs) for transmission, and the OSC signals are transmitted without being amplified through the optical amplifiers. In addition since the OSC signals are used as control signals, the transmission levels of the OSC signals are set at low levels so as not to interfere with the main signals.
On the other hand, repeaterless optical transmission systems are currently receiving attention. Since no repeaters are placed in the transmission lines in the repeaterless optical transmission systems, the construction cost can be reduced, and low-price services are enabled. Therefore, demands for construction of reliable repeaterless optical transmission systems are increasing.
In the conventional repeaterless optical transmission systems, some attempts to increase the transmission distance have been made by raising the transmission level of each optical amplifier provided on the upstream side, or providing a Raman amplifier on the downstream side of each optical-fiber transmission line and injecting strong excitation light into the entire length of each optical-fiber transmission line.
However, since the OSC signals are concerned in the overall control of each system, the OSC signals are required to be transmitted between terminal stations regardless of operations of the optical amplifier in the terminal station on the upstream side or the Raman amplifier in the terminal station on the downstream side, e.g., even when the optical amplifier on the upstream side or the Raman amplifier on the downstream side is not in operation.
That is, in the case where transmission is performed over a long distance in a repeaterless optical transmission system, and main signals are amplified with a gain sufficient for transmission of the main signals over the long distance, the OSC signals are required to be normally transmitted between terminal stations and received by the terminal stations even when the OSC signals are not amplified with the same gain as the main signals. Since it is impossible to raise the transmission levels of the OSC signals more than +10 dBm above levels at which an anti-hazard measure becomes necessary, or lower the reception levels in an O/E (opto-electric conversion) module provided on the receiver side below a minimum reception level, conventionally, long-distance transmission of the OSC signals is difficult.
A technique for preventing deterioration of OSC signals during transmission has been proposed, for example, as disclosed in Japanese Unexamined Patent Publication No. 2000-269902, paragraph Nos. 0033 to 0046 and FIG. 4. According to this technique, the OSC signals are processed by using an optical amplifier having satisfactory noise characteristics and gain efficiency.
In the case where the transmission line is a single mode fiber (SMF), and wavelengths on the shorter-wavelength side of main signals (e.g., around the wavelength of 1,510 nm) are allocated to OSC signals, loss in the OSC signals is great, and therefore it is impossible to increase the transmission distance. Thus, conventionally, systems in which wavelengths around 1,510 nm (located on the shorter-wavelength side of main signals) are allocated to OSC signals have been used in only configurations in which the transmission distance is short, or repeaters are arranged at short intervals.
On the other hand, in the conventional repeaterless optical transmission systems, normally, wavelengths on the longer-wavelength side of the main signals (e.g., wavelengths around 1,625 nm) are allocated to OSC signals. This is because the transmission loss through SMFs are small at the wavelengths around 1,625 nm, and the OSC signals are amplified by stimulated Raman scattering (SRS) by the main signals when the wavelengths on the longer-wavelength side of the main signals (e.g., wavelengths around 1,625 nm) are allocated to the OSC signals. Thus, it is possible to increase the transmission distance of the OSC signals. The SRS is a nonlinear optical phenomenon in which light having a shorter wavelength amplifies light having a longer wavelength.
However, when the gain levels of the main signals vary due to some cause in the above case, the gains in the Raman amplification of the OSC signals also vary, i.e., the optical power levels of the OSC signals also vary. Therefore, in this case, an error is detected by an OSC receiver unit provided in a receiver. In this case, even when no trouble actually occurs in the transmission line, the receiver may recognize that an abnormal condition has occurred in the transmission line.
Incidentally, in repeaterless optical transmission systems, the output power of optical amplifiers provided on the transmitter side is as high as at least 1. W, and the output power of Raman excitation light supplied from the receiver side is as high as at least 1 to 2 W. Therefore, if the optical fiber breaks, and light leaks out, it is very dangerous.
In consideration of the above danger, a function called APSD (Auto Power Shut Down) is provided in the conventional repeaterless optical transmission systems. When a trouble such as a failure in an optical fiber occurs, the APSD function automatically stops the Raman excitation light source and the optical amplifier which outputs the high-power optical signals, and shuts off light emission to the outside of each repeaterless optical transmission system, for the purpose of human body protection and fire prevention.
Conventionally, the APSD function is activated when an error is detected in an OSC signal. That is, when an error in a downstream OSC signal is detected by an apparatus on the downstream side, the apparatus stops the optical output of Raman excitation light from the apparatus, and transmits through an upstream line an OSC signal for a notification of the error. When an apparatus on the upstream side receives the notification of the error, the apparatus on the upstream side stops the output of an optical amplifier in the apparatus on the upstream side.
However, since wavelengths on the longer-wavelength side of the main signals are allocated to the OSC signals in the conventional repeaterless optical transmission systems in which the APSD function is activated in response to detection of an error, the levels of OSC signals are affected by variations in the main signals. Therefore, there is a possibility that the apparatus on the downstream side detects an error even when a failure in an optical fiber does not actually occur, and the APSD function is activated by error when optical communication is normally performed.
Further, according to the conventional technique disclosed in Japanese Unexamined Patent Publication No. 2000-269902, the OSC signals are processed by arranging an optical repeater in a transmission line, and choosing an optical amplifier having satisfactory characteristics in the repeater. Since this technique uses an optical repeater, this technique cannot be applied to the repeaterless optical transmission systems.